Hyper-Rayleigh Scattering from Gold Nanorods

The first hyperpolarizability of gold nanorods with dimensions 64 x 24 x 24 nm(3) is determined by hyper-Rayleigh scattering (HRS) at the fundamental wavelength of 784 nm. For cetyltrimethylammonium bromide (CTAB)-capped nanorods in aqueous solutions, the measured value is beta(Au-CTAB) = (0.88 +/- 0.02) x 10(-24) esu. For the same nanorods after phase transfer into chloroform with replacement of the CTAB capping layer by a dodecanethiol or an octadecanethiol layer, the measured value is only beta(Au-C12) = (0.22 +/- 0.02) x 10(-24) esu and beta(Au-C18) = (0.20 +/- 0.02) x 10(-24) esu using the newly determined water hyperpolarizability reference of beta = 0.087 x 10(-30) esu. In both cases, it is demonstrated through polarization-resolved HRS that the major contribution to the SHG response from these centrosymmetric nanoparticles stems from the surface as a breaking of the centrosymmetry, similarly to what is observed for gold nanospheres. Besides, the reduced hyperpolarizability measured for the gold nanorods with the thioalkane capping layer is attributed to the bond formed between the gold surface atoms and the sulfur atom of the capping layer, similarly to the case of gold nanospheres, too.